Electric heating pad



Oct. 22, 1968 A. E. F. FICKWEILER 3,407,282

ELECTRIC HEATING PAD Filed Nov. 17, 1965 INVENTOR. AUGUST E. F.FI'CKWEILER- United States Patent 3,407,282 ELECTRIC HEATING PAD AugustE. F. Fickweiler, Waddinxveen, Netherlands, as-

signor, by mesne assignments, to Van der Heem N.V., a corporation of theNetherlands Filed Nov. 17, 1965, Ser. No. 508,280

1 Claim. (Cl. 219519) The invention relates to an electric heating padwith a heater cable, comprises a central resistance core and a coaxialsafety sheath, in which the resistance core is connected to the supplyvoltage through the normally open contacts of a relay and the heatercoil and the normally closed contact of a bimetal switch, whilst therelay coil is connected to a relay supply voltage through the safetysheath and a resistor.

Such a heating pad must meet several safety requirements, i.e. that thesupply voltage must be disconnected as soon as overheating of the heatercable threatens. Such an overheating can e.g. be caused by aninterruption in the resistance core, in which case sparks can occur atthe place of the interruption, which sparks can cause an inadmissiblystrong local heating. In a known device, the relay energization isdiscontinued in that case, because the insulating substance between theresistance core and the safety sheath obtains a lower resistance due tothe marked heating, whereby a partial short-circuit between theresistance core and the safety sheath occurs. The safety sheath is soconnected in the energization circuit of the relay, which is connectedto the supply circuit for the resistance core, that such a partialshort-circuit leads to de-energization of the relay. It is adisadvantage of this device, that a very special insulating materialbetween the resistance core and the safety sheath is required, whilstthe operation is rather critical and in practice turns out not to bereliable under all conditions.

It is an object of the invention to provide an electric heating pad, inwhich an interruption of the resistance core leads to immediatedisconnection of the heating pad, without necessitating the use of aspecial insulating material between the resistance core and the safetysheath.

It is a feature of the invention that the primary winding of a currenttransformer is connected in the series circuit of the resistance core,whilst the secondary winding of the current transformer is connected inseries with the normally closed contact of the bimetal switch andtogether therewith constitutes the source of relay supply voltage.

These and various other objects, features, and advantages of theinvention will be better understood from the following description takenin connection with the accompanying drawing, which shows a circuitdiagram of an electric heating pad according to the invention.

In the drawing, the heater cable comprises a central resistance core 1and a coaxial safety sheath 2, which by means of a suitable insulatingmaterial is insulated with respect to the central core 1. Thesecomponents are the only components which are arranged in the heating paditself, whilst the remaining components are all parts of the automaticcontroller for the heating pad.

The resistance core 1 is connected to the supply terminals 8 and 9through the primary winding 3 of a current transformer, the normallyclosed contact 4 and the heater coil 5 of a bimetal relay and thenormally open contacts 6 and 7 of a safety relay. As soon as the relayis operated by external influence, the normally open contacts 6 and 7are closed, so that the resistance core 1 is energized. The currentthrough the primary winding 3 of the current transformer causes avoltage across the secondary winding 10, which voltage causes a currentthrough the resistor 11, the rectifier 12, the safety sheath 3,407,282Patented Oct. 22, 1968 2, a resistor 13, the relay coil 14 and theheater winding 5 and the normally closed contact 4 of the bimetalswitch, wherebythe relay coil 14 is energized and the normally opencontacts 6 and-7 of the relay remain c1osed.- The current through theresistance core 1 not only causesa heating effect of the heating pad,but also of the heater winding 5 of the bimetal switch, so that after.some time the normally closed contact 4 thereof is opened. In that case,the current through the resistance core 1 is discontinued, so that theheat generation in the heating pad is discontinued and the currentthrough the primary winding 3 of the current transformer isdiscontinued, so that the voltage across the secondary winding 10thereof is also discontinued, but the relay coil 14 remains energized,since the full mains voltage is now present across the opened contact 4of the bimetal switch, so that this voltage takes the place of theinitial voltage across the secondary winding 10 of the currenttransformer.

After some time has elapsed, the bimetal switch has cooled down so far,that its normally closed contact 4 is again closed and the resistancecore 1 is again energized. The interval during which the normally closedcontact 4 remains opened is shorter as the ambient temperature of thebimetal switch is lower and its rate of cooling is consequently higher.Thereby, the resistance core 1 is energized during a larger part of thetime, as the ambient temperature of the bimetal switch, that is the roomtemperature, is lower, so that the heat generated in the heating padautomatically adapts itself to the room temperature.

Disconnecting the heating pad can be effected by bring ing the relay inthe inoperative condition mechanically, by interrupting the currentsupply by means of a separate switch or by interrupting the currentthrough the relay coil 14, e.g. by short-circuiting this relay coil bymeans of the shown push-button 15 with normally open contacts.

The resistors 11 and 13 can be so chosen, as is known per se, that theenergization of the relay coil 14 is disrupted as soon as due to amechanical damage of the heater cable, a short-circuit between theresistance core 1 and the safety sheath 2 occurs. The resistors 11 and13 are so chosen, that touching the safety sheath 2 due to a fault ofthe surrounding insulation cannot lead to dangerous consequences, whilstin that case too, the energization of the relay coil 14 is discontinuedand the heating pad is disconnected. Although it would basically bepossible to switch the heating pad on by temporarily bridging thenormally open contacts 6 and 7 of the relay by means of a push-buttonswitch with normally open contacts, this is not very well feasible inpractice, since the contact distance of the opened contacts 6 and 7 mustbe fairly large for safety reasons, so that a very strong energizationof the relay coil 14 would be necessary for having the relay attract itsarmature. This heavy energization current cannot usually be obtained ifthe resistors 11 and 13 are chosen in conformity with the requirementsset forth above, so that it will generally be necessary to have theswitching-on effected by mechanical actuation of the relay. Thecapacitor 16 serves for smoothing the direct voltage which is suppliedto the resistor 13 in series with the relay coil 14.

If an interruption of the resistance core 1 should develop whilst thenormally closed contact 4 of the bimetal switch is opened, the voltageacross that contact is interrupted, so that in that case too, theenergization of the relay coil 14 is discontinued and the normally opencontacts 6 and 7 are opened, whereby the resistance core 1 cannot beswitched on again.

What isclairned is: r V a series circuit of the safety sheath, the relaycoil and the 1. A safety system for an electric heating pad comsecondarywinding of the transformer.

prising a heater cable having a central resistance core and a coaxialsafety sheath, 2. safety relay having at least a nor- References Citedmally open contact and a coil, a temperature responsive 5 UNITED STATESPATENTS switch having a normally closed contact, an electrical supplysource, a transformer, the resistance core cOn- 2914645 11/1959 Wallace219-519 nected to the supply source through the normally open relaycontact, the primary of the transformer and the BERNARD GILHEANY PnmaryExammer normally. closed temperature responsive contact, an en- 10 BELL,Assistant Examiner.

ergy source for the coil of the safety relay consisting of

1. A SAFETY SYSTEM FOR AN ELECTRIC HEATING PAD COMPRISING A HEATER CABLEHAVING A CENTRAL RESISTANCE CORE AND A COAXIAL SAFETY SHEATH, A SAFETYRELAY HAVING AT LEAST A NORMALLY OPEN CONTACT AND A COIL, A TEMPERATURERESPONSIVE SWITCH HAVING A NORMALLY CLOSED CONTACT, AN ELECTRICAL SUPPLYSOURCE, A TRANSFORMER, THE RESISTANCE CORE CONNECTED TO THE SUPPLYSOURCE THROUGH THE NORMALLY OPEN RELAY CONTACT, THE PRIMARY OF THETRANSFORMER AND THE NORMALLY CLOSED TEMPERATURE RESPONSIVE CONTACT, ANENERGY SORUCE FOR THE COIL OF THE SAFETY RELAY CONSISTING OF A SERIESCIRCUIT OF THE SAFETY SHEATH, THE RELAY COIL AND THE SECONDARY WINDINGOF THE TRANSFORMER.